As mentioned above, method for measuring displacement in a laser interferometer used for primary correction of vibration sensor is explained as follows. When a laser beam for measuring vibration displacement (corresponding to the laser passing through angle prism which rotates along with angular accelerometer in the embodiment of the FIG. 1) and a laser beam reflected from a fixed reference side (corresponding to the laser beam lit from the laser head of the embodiment of the FIG. 1, and the fixed reference side can be easily made by other mirrors, etc.) are overlapped with each other and are sent to the interferometer comprising a light sensor, difference of paths, i.e. cosine signal which is cosine component electrically output from the light sensor, and sine signal which is sine component electrically output according to vibration displacement, may be taken.
According to traditional method, after this cosine signal and sine signal are digitalized, phase and vibration amplitude are calculated by using lookup table. More details are as follows. As mentioned above, two signals, which are cosine/sine components electrically output from the light sensor generated by vibration displacement, are converted as u cos(n) and u sin(n) which are discretized value 8-bit (or 12-bit) by using digitalizer. And then, by using lookup table, phase and vibration amplitude are calculated. (n=1, 2, . . . , N, N is the number of all data) FIG. 2 conceptually shows traditional method for measuring phase and vibration amplitude by using lookup table. That is, 16 bits information consisting of two digital values, u cos(n) and u sin(n) is used as input number of lookup table, and phase and vibration amplitude saved in a memory corresponding to the designated number are read. Phase from a lookup table is converted as displacement by using the following formula (A), and consequently, the vibration displacement is measured.
                              d          ⁡                      (            n            )                          =                              λ                          4              ⁢                              π                ·                                  N                  B                                                              ⁢                      θ            ⁡                          (              n              )                                                          (        A        )            d: displacementθ: phaseλ: laser wavelength (m)NB: reflection times of laser for measurement (single reflection NB=1, double reflection NB=2)
However, the existing method for measuring displacement by the cosine/sine signals converted as digital values and the lookup table has the following problems.
(1) If vibration amplitude of cosine and sine signals is different (ratio of vibration amplitude r≠1), if the two signals have quadrature error angle (α≠0), or if there are another DC components (p, q≠0), it meets technical limitation that displacement measurement at the picometer level cannot be realized with the existing method for measuring displacement.(2) There is a disadvantage that a lookup table consisting of u cos(n) and u sin(n) discretized as 8-bit (or 12-bit) requires mass memory which is proportional to the size of the lookup table 22Nb (Nb=the number of bits of AD convertor). For example, when using 12-bit AD convertor, memory having the input number of the least 16 M (16,777,216) is required.
Therefore, requests for new measurement method for vibration displacement have consistently increased, which can overcome the limit of the traditional method.